STM32MP1 - I2C frequency too low

[Dat Tran]

In my device tree, we config 400KHz, but on scope we see only ~ 50Hz, and the clock seems weird. Anyone know why?

&i2c1 {

pinctrl-names = "default", "sleep";

pinctrl-0 = <&i2c1_pins_a>;

pinctrl-1 = <&i2c1_sleep_pins_a>;

i2c-scl-rising-time-ns = <185>;

i2c-scl-falling-time-ns = <20>;

clock-frequency = <400000>;

status = "okay";

/delete-property/dmas;

/delete-property/dma-names;

};

[Erwan SZYMANSKI]

Hello @Community member​ ,

What board do you use ? A custom board ? A discovery board ? MP15 / MP13 ?

Can you check the output of "cat /sys/kernel/debug/clk/clk_summary" and share the ouput of this command ?

The driver i2c-stm32f7.c used to control I2C has dynamic debug available (dev_dbg). Can you activate it or replace dev_dbg by dev_info just to check traces inside the driver at boot / probe time ?

Kind regards,

Erwan.

[Dat Tran]

Hi, Thanks Erwan,

We use custom board, MP151AAC3 MCU.

We did a bit more investigation and found that 'drive-open-drain' causes this issue. When we temporary remove this line in device tree see that the clock is 400KHz as well.

But when we remove drive-open-drain then I2c transaction failed, but the clock on scop looks correctly.

Do you have any idea why when add drive-open-drain the clock become weird?

&pinctrl {

i2c1_pins_a: i2c1-0 {

pins {

pinmux = <STM32_PINMUX('D', 12, AF5)>, /* I2C1_SCL */

<STM32_PINMUX('D', 13, AF5)>; /* I2C1_SDA */

bias-disable;

drive-open-drain; /*Remove this line will make good clocking */

slew-rate = <0>;

};

};

};

[Dat Tran]

Seem change bias-disable to pull up everything work well

[PatrickF · Accepted solution]

Hi @Community member​ 

I confirm both I2C pins should be open-drain (I2C standard to allow clock stretching and acknowledge) with pull-ups.

I think you miss pull-ups on your board (we put 1.5K on our boards).

Internal pull-up might work, but rising time will be quite bad and could create timing issues even at 400kHz if your bus is loaded by multiple devices and/or long wires.

Regards,